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Canvas element in HTML5

黄舟Original: 2016-12-22 15:27:151353browse

The canvas element is used to draw graphics on web pages.

What is Canvas?

HTML 5’s canvas element uses JavaScript to draw images on the web page.

The canvas is a rectangular area that you can control every pixel of.

canvas has multiple ways to draw paths, rectangles, circles, characters, and add images.

Create Canvas element

Add canvas element to HTML 5 page, specify the element's id, width and height:

<canvas id="myCanvas" width="200" height="100"></canvas>

Draw through JavaScript

The canvas element itself has no drawing capabilities. All drawing work must be done inside JavaScript:

<script type="text/javascript"> 
var c=document.getElementById("myCanvas"); 
var cxt=c.getContext("2d"); 
cxt.fillStyle="#FF0000"; 
cxt.fillRect(0,0,150,75); 
</script>

JavaScript uses the id to find the canvas element:

var c=document.getElementById("myCanvas");

Then, create the context object:

var cxt=c.getContext("2d");

getContext("2d") The object is a built-in HTML 5 object with many Ways to draw paths, rectangles, circles, characters, and add images.

Example: Hover the mouse over the rectangle to see the coordinates. Try it yourself. The code is as follows:

<!DOCTYPE HTML>
<html>
<head>
<style type="text/css">
body {
	margin: 0px;
	font-size: 70%;
	font-family: verdana, helvetica, arial, sans-serif;
}
#coordiv {
	float: left;
	width: 199px;
	height: 99px;
	border: 1px solid #c3c3c3
}
</style>
<script type="text/javascript">  
function cnvs_getCoordinates(e) { 
	x=e.clientX; 
	y=e.clientY; 
	document.getElementById("xycoordinates").innerHTML="Coordinates: (" + x + "," + y + ")"; 
} 
function cnvs_clearCoordinates() { 
	document.getElementById("xycoordinates").innerHTML=""; 
} 
</script>
</head>
<body>
<p>把鼠标悬停在下面的矩形上可以看到坐标：</p>
<div id="coordiv" onmousemove="cnvs_getCoordinates(event)" onmouseout="cnvs_clearCoordinates()"></div>
<div id="xycoordinates"></div>
</body>
</html>

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